1. Field of the Invention
The present invention relates generally to cardiac stimulation, and more particularly to implantable leads which simulate or sense electrical activity of the heart. The invention provides a replacement connector which can be connected to an implanted lead in place of a proximal end of the lead.
2. Prior Art
There are generally two types of body implantable leads used with cardiac pacemakers--one which requires surgery to expose the myocardia tissue to which an electrode is affixed and another which is inserted through a body vessel, such as a vein, into the heart where a electrode contacts the endocardial tissue. In the later type, the endocardial lead is often secured to the heart through an endothelial lining by a sharpened helix affixed to a distal end of the lead. When the end of the lead contacts the lining of the heart at a desired location, the lead may be secured in place by rotating the lead, thus screwing the helix into the heart tissue.
When an implantable lead is affixed to the heart tissue, it initially exhibits a certain impedance associated with the character and composition of the lead, the location and quality of fixation, and other factors. Over time, the heart tissue reacts to the presence of the foreign body, forming a fibrosis in the tissue near the electrode. Shortly after implantation, therefore, the impedance associated with an implanted electrode rises from an initial value to a peak value. Thereafter, the impedance associated with the electrode again falls to a lower value as the fibrosis stabilizes and eventually a relatively stable, long-term impedance value is attained.
This phenomenon is well known. One consequence of this condition is that initial settings for implanted pacers must be higher than the measured impedance at the time of implantation. An attending physician would expect the impedance to rise over time and would, therefore, adjust the pacer to produce pulses of higher energy in order to insure an appropriate response in the heart during the period of highest impedance. Frequently, the pacer is not adjusted after the impedance falls to its stable long-term value, resulting in an unnecessary use of pacer power. Higher power consumption results in more rapid battery depletion, which eventually requires that the pacer itself be replaced sooner than would be otherwise necessary. It is desireable, therefore, for a physician to be able to use a lead which has already stabilized to its long-term impedance, whenever the pacer is replaced. Use of a stabilized lead can permit the attending physician to more accurately adjust both sensing and stimulating parameters to conserve energy and to assure reliable long-term performance of the implanted pacer.
The replacement pacer, however, may not be the same type as the original pacer. Over the expected life of the pacer, on the order of ten years, it is to be expected that technology and manufacturing will have changed. New options in pacing technology have become and will become available. Moreover, the patient's condition may have changed, making it desireable to incorporate different features in the replacement pacer, which were not necessary in the original pacer.
Because it is quite likely that the electrical connections on the replacement pacer would not be identical to the original pacer, it is desireable to provide a means whereby a distal end of the implanted lead could be left in its stabilized condition near or on the heart tissue, while the proximal end of the lead is replaced with an end compatible with the replacement pacer.